The young embedded disk L1527 IRS: constraints on the water snowline and cosmic ray ionization rate from HCO+ observations

TL;DR

This study uses high-resolution ALMA observations of HCO+ isotopologues in the young disk L1527 IRS to constrain the water snowline location and cosmic ray ionization rate, highlighting the potential of chemical imaging in planet formation studies.

Contribution

It demonstrates how HCO+ observations combined with physical modeling can estimate the snowline position and cosmic ray ionization rate in a young protostellar disk.

Findings

Snowline located between 1.8 and 4.1 au.

Cosmic ray ionization rate constrained to 10^{-18} s^{-1}.

HCO+ emission modeling matches observations with adjusted ionization rate.

Abstract

The water snowline in circumstellar disks is a crucial component in planet formation, but direct observational constraints on its location remain sparse due to the difficulty of observing water in both young embedded and mature protoplanetary disks. Chemical imaging provides an alternative route to locate the snowline, and HCO$^+$ isotopologues have been shown to be good tracers in protostellar envelopes and Herbig disks. Here we present $\sim$0.5$^{\prime\prime}$ resolution ($\sim$35 au radius) Atacama Large Millimeter/submillimeter Array (ALMA) observations of HCO$^+$ $J=4-3$ and H$^{13}$CO$^+$ $J=3-2$ toward the young (Class 0/I) disk L1527 IRS. Using a source-specific physical model with the midplane snowline at 3.4 au and a small chemical network, we are able to reproduce the HCO$^+$ and H$^{13}$CO$^+$ emission, but for HCO$^+$ only when the cosmic ray ionization rate is lowered to $10^{-18}$ s$^{-1}$. Even though the observations are not sensitive to the expected HCO$^+$ abundance drop across the snowline, the reduction in HCO$^+$ above the snow surface and the global temperature structure allow us to constrain a snowline location between 1.8 and 4.1 au. Deep observations are required to eliminate the envelope contribution to the emission and to derive more stringent constraints on the snowline location. Locating the snowline in young disks directly with observations of H$_2$O isotopologues may therefore still be an alternative option. With a direct snowline measurement, HCO$^+$ will be able to provide constraints on the ionization rate.